We expect that in a future level of CSS there will be new
properties and values defined for speech output. Therefore
CSS 2.1 reserves the 'speech' media type (see chapter 7, "Media types"), but does not yet
define which properties do or do not apply to it.

The properties in this appendix apply to a media type 'aural', that
was introduced in CSS2. The type 'aural' is now deprecated.

The aural rendering of a document, already commonly used by the
blind and print-impaired communities, combines speech synthesis and
"auditory icons." Often
such aural presentation occurs by converting the document to plain
text and feeding this to a screen reader -- software or hardware that
simply reads all the characters on the screen. This results in less
effective presentation than would be the case if the document
structure were retained. Style sheet properties for aural presentation
may be used together with visual properties (mixed media) or as an
aural alternative to visual presentation.

Besides the obvious accessibility advantages, there are other large
markets for listening to information, including in-car use, industrial
and medical documentation systems (intranets), home entertainment, and
to help users learning to read or who have difficulty reading.

When using aural properties, the canvas consists of a three-dimensional physical
space (sound surrounds) and a temporal space (one may specify sounds
before, during, and after other sounds). The CSS properties also
allow authors to vary the quality of synthesized speech (voice type,
frequency, inflection, etc.).

This will direct the speech synthesizer to speak headers in a voice
(a kind of "audio font") called "paul", on a flat tone, but in a very
rich voice. Before speaking the headers, a sound sample will be played
from the given URL. Paragraphs with class "heidi" will appear to come
from front left (if the sound system is capable of spatial audio), and
paragraphs of class "peter" from the right. Paragraphs with class
"goat" will be very soft.

Volume refers to the
median volume of the waveform. In other words, a highly inflected
voice at a volume of 50 might peak well above that. The overall values
are likely to be human adjustable for comfort, for example with a
physical volume control (which would increase both the 0 and 100
values proportionately); what this property does is adjust the dynamic
range.

Percentage values are calculated relative to the inherited value,
and are then clipped to the range '0' to '100'.

silent

No sound at all. The value '0' does not mean
the same as 'silent'.

x-soft

Same as '0'.

soft

Same as '25'.

medium

Same as '50'.

loud

Same as '75'.

x-loud

Same as '100'.

User agents should allow the values corresponding to '0' and '100'
to be set by the listener. No one setting is universally applicable;
suitable values depend on the equipment in use (speakers, headphones),
the environment (in car, home theater, library) and personal
preferences. Some examples:

A browser for in-car use has a setting for when there is lots of
background noise. '0' would map to a fairly high level and '100' to a
quite high level. The speech is easily audible over the road noise but
the overall dynamic range is compressed. Cars with better
insulation might allow a wider dynamic range.

Another speech browser is being used in an apartment, late at
night, or in a shared study room. '0' is set to a very quiet level and
'100' to a fairly quiet level, too. As with the first example, there
is a low slope; the dynamic range is reduced. The actual volumes are
low here, whereas they were high in the first example.

In a quiet and isolated house, an expensive hi-fi home theater
setup. '0' is set fairly low and '100' to quite high; there is wide
dynamic range.

The same author style sheet could be used in all cases, simply by
mapping the '0' and '100' points suitably at the client side.

This property specifies whether text will be rendered aurally and
if so, in what manner. The possible values are:

none

Suppresses aural rendering so that the
element requires no time to render. Note, however, that
descendants may override this value and will be spoken. (To
be sure to suppress rendering of an
element and its descendants, use the
'display' property).

normal

Uses language-dependent pronunciation rules for rendering
an element and its children.

spell-out

Spells the text one letter at a time (useful for acronyms and
abbreviations).

Note the difference between an element whose 'volume' property has a value of
'silent' and an element whose 'speak' property has the value 'none'.
The former takes up the same time as if it had been spoken, including
any pause before and after the element, but no sound is generated. The
latter requires no time and is not rendered (though its descendants
may be).

Refers to the inverse of the value of the
'speech-rate' property.
For example, if the speech-rate is 120 words per minute
(i.e., a word takes half a second, or 500ms) then a 'pause-before' of 100% means a
pause of 500 ms and a 'pause-before' of 20% means
100ms.

The URI must designate an auditory icon resource. If the URI resolves to something other than an audio file, such as an image, the resource should be ignored and the property treated as if it had the value 'none'.

If a user agent cannot render an auditory icon (e.g., the user's
environment does not permit it), we recommend that it produce an
alternative cue.

Please see the sections on the :before and :after
pseudo-elements for information on other content generation
techniques. 'cue-before' sounds and 'pause-before' gaps are inserted
before content from the ':before' pseudo-element. Similarly,
'pause-after' gaps and 'cue-after' sounds are inserted after content
from the ':after' pseudo-element.

The sound designated by this <uri> is played
as a background while the element's content is spoken.

mix

When present, this keyword means that
the sound inherited from the parent element's 'play-during' property continues
to play and the sound designated by the <uri> is mixed with it. If
'mix' is not specified, the element's background sound replaces
the parent's.

repeat

When present, this keyword means that the sound will repeat if it
is too short to fill the entire duration of the element. Otherwise,
the sound plays once and then stops. This is similar to the 'background-repeat'
property. If the sound is too long for the element, it is clipped once
the element has been spoken.

auto

The sound of the parent element continues to play
(it is not restarted, which would have been the case if this property
had been inherited).

none

This keyword means that there is silence. The sound of the
parent element (if any) is silent during the current element and
continues after the current element.

Spatial audio is an important stylistic property for aural
presentation. It provides a natural way to tell several voices apart,
as in real life (people rarely all stand in the same spot in a
room). Stereo speakers produce a lateral sound stage. Binaural
headphones or the increasingly popular 5-speaker home theater setups
can generate full surround sound, and multi-speaker setups can create
a true three-dimensional sound stage. VRML 2.0 also includes spatial
audio, which implies that in time consumer-priced spatial audio
hardware will become more widely available.

Position is described in terms of an angle
within the range '-360deg' to '360deg'.
The value '0deg' means directly ahead in the center of the sound
stage. '90deg' is to the right, '180deg' behind, and '270deg' (or,
equivalently and more conveniently, '-90deg') to the left.

left-side

Same as '270deg'. With 'behind', '270deg'.

far-left

Same as '300deg'. With 'behind', '240deg'.

left

Same as '320deg'. With 'behind', '220deg'.

center-left

Same as '340deg'. With 'behind', '200deg'.

center

Same as '0deg'. With 'behind', '180deg'.

center-right

Same as '20deg'. With 'behind', '160deg'.

right

Same as '40deg'. With 'behind', '140deg'.

far-right

Same as '60deg'. With 'behind', '120deg'.

right-side

Same as '90deg'. With 'behind', '90deg'.

leftwards

Moves the sound
to the left, relative to the current angle.
More precisely, subtracts 20 degrees.
Arithmetic is carried out modulo 360 degrees. Note that
'leftwards' is more accurately described as "turned
counter-clockwise," since it always subtracts 20 degrees,
even if the inherited azimuth is already behind the listener (in which
case the sound actually appears to move to the right).

rightwards

Moves the sound
to the right, relative to the
current angle. More precisely, adds 20 degrees. See 'leftwards'
for arithmetic.

This property is most likely to be implemented by mixing the same
signal into different channels at differing volumes. It might also
use phase shifting, digital delay, and other such techniques to
provide the illusion of a sound stage. The precise means used to
achieve this effect and the number of speakers used to do so are
user agent-dependent; this property merely identifies the desired end
result.

If spatial-azimuth is specified and the output device cannot
produce sounds behind the listening position, user agents
should convert values in the rearwards hemisphere to forwards
hemisphere values. One method is as follows:

Specifies the elevation as an angle, between '-90deg' and '90deg'.
'0deg' means on the forward horizon, which loosely means level with
the listener. '90deg' means directly overhead and '-90deg' means directly
below.

below

Same as '-90deg'.

level

Same as '0deg'.

above

Same as '90deg'.

higher

Adds 10 degrees to the current elevation.

lower

Subtracts 10 degrees from the current elevation.

The precise means used to achieve this effect and the
number of speakers used to do so are undefined. This property merely
identifies the desired end result.

Names of specific voices may be quoted, and indeed must be quoted
if any of the words that make up the name does not conform to the
syntax rules for identifiers. It is also
recommended to quote specific voices with a name consisting of more
than one word. If quoting is omitted, any white space characters before and
after the voice family name are ignored and any sequence of white space
characters inside the voice family name is converted to a single space.

Specifies the average pitch (a frequency) of the speaking voice. The
average pitch of a voice depends on the voice family. For example,
the average pitch for a standard male voice is around 120Hz,
but for a female voice, it's around 210Hz.

These values do not map to absolute frequencies since
these values depend on the voice family. User agents should map
these values to appropriate frequencies based on the voice family
and user environment. However, user agents must map these values in
order (i.e., 'x-low' is a lower frequency than 'low', etc.).

Specifies variation in average pitch. The perceived pitch of a
human voice is determined by the fundamental frequency and typically
has a value of 120Hz for a male voice and 210Hz for a female voice.
Human languages are spoken with varying inflection and pitch; these
variations convey additional meaning and emphasis. Thus, a highly
animated voice, i.e., one that is heavily inflected, displays a high
pitch range. This property specifies the range over which these
variations occur, i.e., how much the fundamental frequency may deviate
from the average pitch.

Specifies the height of "local peaks" in the intonation contour
of a voice. For example, English is a stressed
language, and different parts of a sentence are assigned primary,
secondary, or tertiary stress. The value of 'stress' controls the amount of
inflection that results from these stress markers. This property is a
companion to the 'pitch-range' property and is
provided to allow developers to exploit higher-end auditory displays.

A value, between '0' and '100'. The meaning of values
depends on the language being spoken. For example,
a level of '50' for a
standard, English-speaking male voice (average pitch = 122Hz), speaking
with normal intonation and emphasis would have a different
meaning than '50' for an Italian voice.

When a table is spoken by a speech generator, the relation between
the data cells and the header cells must be expressed in a different
way than by horizontal and vertical alignment. Some speech browsers
may allow a user to move around in the 2-dimensional space, thus
giving them the opportunity to map out the spatially represented
relations. When that is not possible, the style sheet must specify at
which points the headers are spoken.

This property specifies whether table headers
are spoken before every
cell, or only before a cell when that cell is associated with a
different header than the previous cell. Values have
the following meanings:

once

The header is spoken one time, before a series of
cells.

always

The header is spoken before every pertinent cell.

Each document language may have different mechanisms that allow
authors to specify headers. For example, in HTML 4 ([HTML4]),
it is possible to specify header information with three different
attributes ("headers", "scope", and "axis"), and the specification
gives an algorithm for determining header information when these
attributes have not been specified.

Image of a table with header cells ("San
Jose" and "Seattle") that are not in the same column or row as the
data they apply to.

This HTML example presents the money spent on meals, hotels and
transport in two locations (San Jose and Seattle) for successive
days. Conceptually, you can think of the table in terms of an
n-dimensional space. The headers of this space are: location, day,
category and subtotal. Some cells define marks along an axis while
others give money spent at points within this space. The markup
for this table is:

By providing the data model in this way, authors make it
possible for speech enabled-browsers to explore the table in
rich ways, e.g., each cell could be spoken as a list, repeating the
applicable headers before each data cell: